The present invention relates generally to microwave cooking appliances and more particularly to microwave oven cooking cavity excitation systems for promoting time-averaged uniformity of microwave energy distribution in the cooking cavity.
A problem of long standing in microwave oven appliances has been the non-uniform spatial distribution of microwave energy in the cooking cavity. This non-uniform energy distribution results in hot spots and cold spots at different locations in the cavity. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity which do not readily conduct heat from the areas which are heated by the microwave energy to those areas which are not. One example of such a food is cake.
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried. One common approach is the use of a device known as a mode stirrer which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwaves) or to conserve space within the cooking cavity the stirrer may be mounted within a recess formed in one of the cooking cavity walls, normally the top. The function of the mode stirrer is to continually alter the mode pattern within the cooking cavity.
Another approach to the problem of non-uniform energy distribution is disclosed in commonly-assigned U.S. Pat. No. 4,336,434, issued June 22, 1982 to Matthew S. Miller, entitled "Microwave Oven Cavity Excitation System Employing Circularly Polarized Beam Steering for Uniformity of Energy Distribution and Improved Impedance Matching." The disclosed Miller microwave oven cavity excitation system introduces circularly polarized electromagnetic wave energy into a cooking cavity through a pair of feed points appropriately phased to provide a concentrated beam. The relative phasing of the feed points is varied as a function of time to steer the concentrated beam to sweep the interior of the cavity, thereby improving the time-averaged energy distribution within the cooking cavity. Further, the disclosure of the Miller patent points out that as a result of the circular polarization, standing waves in the direction of one of the cavity dimensions are minimized and the amount of energy reflected back to the generator is reduced. The Miller patent also shows how various forms of coupling apertures or slots in a rectangular waveguide can be located with respect to the waveguide so as to radiate a circularly polarized electromagnetic field.
Another approach involving a modification of the above-identified Miller patent is disclosed in commonly-assigned U.S. Pat. No. 4,324,968, issued Apr. 13, 1982 to Peter H. Smith and entitled "Microwave Oven Cavity Excitation System Providing Controlled Electric Field Shape for Uniformity of Energy Distribution." The Smith oven cavity excitation system provides a coupling aperture such as an X slot for radiating microwave energy from a feed wave guide into the adjacent cooking cavity, which slot is effectively controllably and selectively moved with respect to the wave guide centerline with the result that the sectional shape of the resulting field, viewed for example in the plane of the food supported on a conventionally located shelf, changes from circular to elliptical with the degree and orientation of the ellipse depending upon the direction and degree of movement of the coupling aperture with respect to the waveguide centerline. Rather than physically moving the aperture, a device is provided for varying the electrical position of the coupling aperture with respect to the centerline of the waveguide as a function of time.
Yet another approach to the problem is disclosed in commonly-assigned, copending application, Ser. No. 363,705, filed Mar. 30, 1982 by Dills et al, and entitled "Microwave Oven with Dual Feed Excitation System." The disclosed Dills et al excitation system employs a rotating antenna in combination with a slotted feed arrangement which interacts so as to improve the efficiency and uniformity of heating within the cavity. The rotating antenna radiates a dynamic field from the top wall of the cavity and the slotted bottom feed radiates a static field from a radiating chamber extending along the bottom cavity wall and having an array of radiating slots formed along the top face of the chamber. The slots are arranged to establish a substantially stationary radiation pattern in the cavity which complements the average radiation pattern of the antenna by filling those portions of the antenna pattern of relatively low energy density. Since the impedance of the antenna load is a function of the angular orientation of the antenna in the cavity, this impedance varies as the antenna rotates. The antenna and the chamber are both fed from a common source; thus, the proportion of total energy delivered to the chamber fluctuates as the antenna load impedance fluctuates, causing the intensity of the output of the radiating chamber slots to fluctuate accordingly. This interaction of the dynamic rotating antenna and the static radiating chamber results in a more uniform energy distribution throughout the cavity when time-averaged over the cooking period.
In addition to the above-referenced Dills et al application, other microwave oven excitation systems employing slotted feed arrangements known in the microwave art include U.S. Pat. No. 4,019,009 to Kusonoki et al; U.S. Pat. No. 2,704,802 to Blass et al; and U.S. Pat. No. 3,810,248 to Risman et al. The slotted feed arrangement of the Kusonoki et al type uses surface wave phenomena for near field heating. Such an arrangement tends to primarily heat the portion of the load nearest the slots and works less well for relatively thin slot loads. For other types of loads, the surface waves are supplemented by energy radiated into the cavity from the top or sides. Slotted feed arrangements such as that of Blass et al and Risman et al tend to create standing waves in the cavity with resultant cold spots at the nodes of the standing waves. Commonly-assigned, U.S. Pat. No. 4,354,083 to James E. Staats, provides an example of a dual feed system using slotted radiators in the top and bottom cavity walls. A shelf is positioned immediately above the bottom slots to heat food supported on the shelf from the bottom by use of near field heating effect, while the slots radiate microwave energy to illuminate the upper portion of the food load. In each of these slotted feed arrangements the essentially static field is supported in the cavity by the slotted feed radiators.
While the various approaches to the problem of non-uniform energy distribution in microwave cavities summarized hereinbefore have achieved varying degrees of success in improving cooking performance, it will be appreciated that the achievement of time-averaged uniformity of energy distribution is a formidable consideration in the development of practical microwave ovens.
It is therefore an object of the present invention to provide a microwave oven excitation system which provides improved uniformity of time-averaged energy distribution in the oven cavity to more effectively cook even those foods having low thermal conductivity properties with an excitation system of relatively simple and inexpensive construction and with a minimum of mechanically moving parts for reduced cost and greater reliability of operation.